Chapter 9Molecular Geometry

and Bonding Theories

Molecular Shapes

• Shape of a molecule plays an important role in its reactivity• The number of bonding and nonbonding electron pairs is

used to predict the shape of the molecule

Lewis structure of carbon tetrachloride, CCl4

Fig 9.1 Tetrahedral geometry of CCl4

Fig 9.2 Some common molecular shapes

Fig 9.3 Shapes of ABn molecules

Fig 9.3 Derivatives from ABn geometry

CH4 H2ONH3

What Determines the Shape of a Molecule?

• Electron pairs, whether they be bonding or nonbonding, repel each other

• By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule

Electron Domains

• Electron pairs are referred to as electron domains.

• In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain.

• Central atom, A, in this molecule, has four electron domains

Valence shell electron pair repulsion (VSEPR) model:

The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.

Fig 9.5 Balloon analogy for electron domains

Table 9.1

Electron-domaingeometries as afunction of the

number of electrondomains

Fig 9.6 The molecular geometry of NH3

• Electron-domain geometry is often not the shape of the molecule, however.

• Molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.

The molecular geometry of CO2

The molecular geometry of O3

Linear; 180°

Bent; 120°

Cl ClBe

2 atoms bonded to central atom0 lone pairs on central atom

BeCl2

BF3

CH4

equatorial

axial

axial

PCl5

SF6

Linear Electron Domain

• In the linear domain, there is only one molecular geometry: linear

• NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain

Table 9.2

Trigonal Planar Electron Domain

• There are two molecular geometries:

– Trigonal planar, if all the electron domains are bonding

– Bent, if one of the domains is a nonbonding pair

Table 9.2

Tetrahedral Electron Domain

• There are three molecular geometries:– Tetrahedral, if all are bonding pairs,– Trigonal pyramidal if one is a nonbonding pair,– Bent if there are two nonbonding pairs.

Table 9.2

Nonbonding Pairs and Bond Angles

• Nonbonding pairs are physically larger than bonding pairs.

• Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

Multiple Bonds and Bond Angles

• Double and triple bonds place greater electron density on one side of the central atom than do single bonds:

• Therefore, they also affect bond angles.

Trigonal Bipyramidal Electron Domain

• There are two distinct positions in this geometry:

– Axial

– Equatorial

Trigonal Bipyramidal Electron Domain

Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.

SF4

Trigonal Bipyramidal Electron Domain

• There are four distinct molecular geometries in this domain:

– Trigonal bipyramidal

– Seesaw

– T-shaped

– Linear

Table 9.3

Octahedral Electron Domain

• All positions are equivalent in the octahedral domain.

• There are three molecular geometries:

– Octahedral

– Square pyramidal

– Square planar

Table 9.3

Predicting Molecular Geometry1. Draw Lewis structure for molecule.

2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom.

3. Use VSEPR to predict the geometry of the molecule.

What are the molecular geometries of SO2 and SF4?

SO O

AB2E

bent

S

F

F

F F

AB4E

distortedtetrahedron